Substance Use Disorders

Substance use disorders generally involve behaviour patterns in which individuals continue to use a substance despite having problems caused by its use.

The substances involved tend to be members of the 10 classes of drug that typically cause substance-related disorders:

  • Alcohol.
  • Antianxiety and sedative drugs.
  • Caffeine.
  • Cannabis (including marijuana and synthetic cannabinoids).
  • Hallucinogens (including LSD, phencyclidine, and psilocybin).
  • Inhalants (such as paint thinner and certain glues).
  • Opioids (including fentanyl, morphine, and oxycodone).
  • Stimulants (including amphetamines and cocaine).
  • Tobacco.
  • Other (including anabolic steroids and other commonly abused substances).

These substances all directly activate the brain’s reward system and produce feelings of pleasure.

The activation may be so strong that individuals intensely crave the substance. They may neglect normal activities to obtain and use the drug.

Substance use disorders can develop whether or not a drug:

  • Is legal;
  • Is socially acceptable; or
  • Has an accepted medical use (with or without a prescription).

Details about specific drugs and their effects are discussed elsewhere in the website.

The terms “addiction,” “abuse,” and “dependence” have traditionally been used in regard to individuals with substance use disorders.

However, those terms are all too loosely and variably defined to be very useful and also are often used judgmentally.

Thus, medical professionals now prefer to use the more comprehensive and less negative term “substance use disorder.”

Recreational and Illicit Substance Use

Use of illegal drugs, although problematic from a legal standpoint, does not always involve a substance use disorder.

On the other hand, legal substances, such as alcohol and prescription drugs (and marijuana in an increasing number of states in the United States and the United Kingdom), may be involved in a substance use disorder.

Problems caused by use of prescription and illegal drugs cut across all socioeconomic groups.

Recreational drug use has existed in one form or another for centuries.

Individuals have used drugs for a variety of reasons, including:

  • To alter or enhance mood;
  • As part of religious ceremonies;
  • To gain spiritual enlightenment; and/or
  • To enhance performance.

Individuals who take drugs recreationally may take them occasionally in relatively small doses, often without doing themselves harm.

That is, users do not develop drug withdrawal, and the drug does not physically harm them (at least in the short term).

Drugs usually considered recreational include:

  • Opium;
  • Alcohol;
  • Nicotine;
  • Marijuana;
  • Caffeine;
  • Hallucinogenic mushrooms (see also Mushroom (Toadstool) Poisoning); and
  • Cocaine.

Many recreational drugs are considered “natural” because they are close to their plant origin. They contain a mixture of low-concentration psychoactive ingredients rather than isolated psychoactive compounds.

Recreational drugs are usually taken by mouth or inhaled.


Individuals usually progress from experimentation to occasional use and then to heavy use and sometimes to a substance use disorder.

This progression is complex and only partially understood.

The process depends on interactions between the drug, user, and setting (discussed below).


Drugs in the 10 classes vary in how likely they are to cause a substance use disorder.

The likelihood is termed addiction liability.

Addiction liability depends upon a combination of factors including:

  • How the drug is used;
  • How strongly the drug stimulates the brain’s reward pathway;
  • How quickly the drug works; and/or
  • The drug’s ability to induce tolerance and/or symptoms of withdrawal.

In addition, substances that are legally and/or readily available, such as alcohol and tobacco, are more likely to be used first.

As individuals continue to use a substance, they often see less risk in using it and may begin to increase their use and/or experiment with other substances.

Individuals’s perception of risk also may be influenced by the social and legal consequences of use.

During treatment of medical illness or following surgical or dental procedures, individuals are routinely prescribed opioids.

If individuals do not take the whole amount prescribed, the drugs sometimes end up in the hands of people who wish to use them recreationally.

Because the use of these drugs for non-medical purposes has become such a large problem, many health care providers (particularly in the US) have responded by:

  • Prescribing lower amounts of opioid drugs;
  • Encouraging individuals to safely store or dispose of any leftover drugs; and/or
  • Expanding prescription take-back programmes.


Factors in users that may predispose to a substance use disorder include:

  • Physical characteristics;
  • Personal characteristics; and
  • Circumstances and disorders.

Physical characteristics likely include genetic factors, although researchers have yet to find more than a few biochemical and/or metabolic differences between individuals who develop a substance use disorder and those who do not.

Individuals with low levels of self-control (impulsivity) or high levels of risk-taking and novelty-seeking behaviours may have an increased risk of developing a substance use disorder.

However, there is little scientific evidence to support the concept of the addictive personality that has been described by some behavioural scientists.

A number of circumstances and coexisting disorders appear to increase the risk of a substance use disorder. For example:

  • Individuals who are sad, emotionally distressed, or socially alienated may find temporary relief from drug use, which can lead to increased use and sometimes to a substance use disorder.
  • Individuals with other, unrelated mental disorders such as anxiety or depression are at increased risk of developing a substance use disorder.
    • Medical professionals use the term “dual diagnosis” to refer to individuals who have both a mental disorder and a substance use disorder.
  • Individuals with chronic pain often require opioid drugs for relief.
    • Some of these individuals later develop a substance use disorder.

However, in many of these individuals, nonopioid drugs and other treatments do not adequately relieve pain and suffering.


Cultural and social factors are very important in initiating and maintaining (or relapsing to) substance use.

Watching family members (e.g. parents or older siblings) and peers using substances increases the risk that individuals will begin using substances.

Peers are a particularly powerful influence among adolescents.

Individuals who are trying to stop using a substance find it much more difficult if they are around others who also use that substance.

Medical professionals may inadvertently contribute to harmful use of psychoactive drugs by overzealously prescribing them to relieve stress.

Many social factors, including mass media, contribute to individuals’ expectation that drugs should be used to relieve all distress.

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